The present invention relates to the fabrication of laminate materials and particularly is directed to a method for validating the ordering of the plies of laminate material. The method according to this invention contemplates maintenance of a demonstrable record, in the form of a trim sample, as evidence of the ordering of the sequential plies of a laminate material so that the integrity of a selected part may later be demonstrated, if necessary. While the present invention may be applied to the fabrication of any laminate material wherein sequential ply ordering is desirable or critical, this invention finds particular applicability and usefulness in the construction of parts from composite materials, and in particular, when advanced composite materials are used.
A composite material is typically created by the combination of two or more materials which include a reinforcing material, such as a fibrous material, bound together by a resin. Composites can include a variety of material layers in addition to the fiber and resin components. The term "advanced composites", however, is generally understood to refer to composites made of high strength, high-modulus fibers of carbon, aramid or boron bound together by a resin matrix material. The matrix used for a composite may include organic materials, metal, carbon or ceramics. Organic materials comprise the most common composite matrices, and these organics are usually polymer resin matrices which may be either a thermoset polymer resin or a thermo-plastic.
The construction of composite materials is typically one of two types, laminates and sandwiches. Sandwiched material is formed by placing a low density core material between two plies of composite material while laminate materials are plies of composite materials that are bonded together. In each case, these materials offer advantages of strength, weight and rigidity. The present invention is specifically directed to composite laminates as opposed to sandwiched material and to the relative ordering of plies within the bonded composite layers.
When part of an article is fabricated utilizing an advanced composite material, the procedure starts with a partially cured composite ply called a prepreg stock sheet wherein the fibrous material is set in the matrix so that the fibers are aligned parallel to one another. A pattern of the product's shape is cut out and the prepreg plies are then stacked into the desired lamiate geometry. The set of assembled plies is then placed under heat and pressure (autoclave) to be cured so that the laminate plies bond to one another. The direction in which the fibers run in the pe-cured laminate ply is called the grain, and it is often desirable that the sequential plies of a final part have a specific preselected grain orientation with respect to one another. Indeed, engineering specifications may call for a particular series of relative angle orientations among the sequential plies and, when each of the plies is cut out from the stock material, it is rotated to the desired grain direction. Thus, assembly of the plies into the article requires critical attention so that ordering is maintained and the correct sequence obtained.
A problem is encountered in the stacking of the laminate plies prior to autoclaving since there is not convenient way to visually ascertain grain angle to insure that the correct ordering is present. Further, once the laminated part is cured, it is extremely difficult to determine the ordering of the ply layers to insure that the sequential ordering of the plies was followed according to the engineering specifications without error. Further, since validation is difficult, cost of manufacture of complex systems of ply to ply rotation become so expensive that few engineering specifications call for complex rotational callouts despite the advantages to be gained. Thus, there is a great need for a method that will allow for the visual validation of the ply ordering and wherein a permanent record of the validity of ply ordering may be maintained. This method must further be employable by current fabrication techniques which include computer-aided design methods employed with automated tool cutting.